Systems and Methods of Driving Multiple Outputs

ABSTRACT

Systems and methods of driving multiple outputs are provided in which a single inductor may be used to drive multiple output such as independent strings of LEDs or white LEDs (WLEDs). In an example embodiment, a boost DC to DC converter may be used with a single inductor to drive multiple outputs. In an example embodiment, the error voltage of each of the multiple outputs is sampled during each cycle of the DC to DC converter and the largest error voltage is determined for that cycle. Power from the DC to DC converter is then supplied to that output during that cycle.

TECHNICAL FIELD

The present disclosure is generally related to electronics and, moreparticularly, is related to driving multiple outputs.

BACKGROUND

Switching power supplies are used to drive many types of loads. Attimes, it is desired to drive multiple loads using a single switchingpower supply requiring only a single inductor. This is particularlydesired when driving multiple light emitting diode (LED) loads as itsaves board space and money. Instead of using a power supply module todrive each string of LEDs, a single power supply module may be used todrive multiple LED strings. But as with many load types, it may beimportant to properly regulate the power delivered to the load.

A commonly used switching power supply topology uses current modecontrol. Current mode control, as usually implemented in switching powersupplies, actually senses and controls peak inductor current. This maygive rise to many serious problems, including poor noise immunity, aneed for slope compensation, and peak-to-average current errors which aninherently low current loop gain cannot correct. Average current modecontrol eliminates these problems and may be used effectively to controlcurrents other than inductor current, allowing a much broader range oftopological application.

Current mode control is a two-loop system in which the switching powersupply inductor is located within the inner current control loop. Thissimplifies the design of the outer voltage control loop and improvespower supply performance in many ways, including better dynamics. Theobjective of this inner loop is to control the state-space averagedinductor current, but in practice the instantaneous peak inductorcurrent is the basis for control. In many designs, switch current, whichis equal to inductor current during the “on” time of the switch, isoften sensed.

In a conventional switching power supply employing a buck derivedtopology, the inductor is located in the output. Current mode controlthen is actually controlling the output current, resulting in manyperformance advantages. On the other hand, in a boost topology, theinductor is located at the input. Current mode control then controlsinput current. The technique of average current mode control introducesa high gain integrating current error amplifier (CA) into the currentloop. A voltage across a sense resistor represents the desired currentprogram level. The voltage across the current sense resistor representsactual inductor current. The difference, or current error, is amplifiedand compared to a large amplitude sawtooth (oscillator ramp) at the PWMcomparator inputs. The gain-bandwidth characteristic of the current loopcan be tailored for optimum performance by the compensation networkaround the CA. The average current mode method can be used to sense andcontrol the current in any circuit branch. Thus it can control outputcurrent with boost topologies, for example.

LEDs are semiconductors with light-emitting junctions designed to uselow-voltage, constant current DC power to produce light. LEDs may use anaverage current, boost mode switching power supply. LEDs have polarityand, therefore, current only flows in one direction. Driving LEDs isrelatively simple and, unlike fluorescent or discharge lamps, they donot require an ignition voltage to start. However, too little currentand voltage will result in little or no light, and too much current andvoltage can damage the light-emitting junction of the LED diode.

When lighting designers arrange a series of LED strings in applicationssuch as street lights or industrial lights, each string have been drivenat a consistent current by an individual LED driver. However, the outputvoltage often varies due to differences in the manufacturing of theLEDs. To compensate, LED drivers may be configured to providehigher-than-needed voltage to ensure proper operation of each LEDstring. Too much voltage, though, can waste power.

With a typical LED forward voltage vs. forward current profile, for agiven temperature, a small change in forward voltage produces adisproportionately large change in forward current. In addition, theforward voltage required to achieve a desired light output can vary withLED die size, LED die material, LED die lot variations, and temperature.

As LEDs heat up, the forward voltage drops and the current passingthrough the LED increases. The increased current generates additionalheating of the junction. If nothing limits the current, the junction mayfail due to the heat. This phenomenon is referred to as thermal runaway.

Light output of LED light sources increases with increasing drivecurrent. However the efficiency, expressed in lumens per watt, isadversely affected. Drive currents may be chosen at any current up tothe maximum recommended current for the specific LED light source used.Driving LED light sources above the maximum recommended currents mayresult in lower lumen maintenance or, with excessive currents,catastrophic failure.

In non-dimming applications, a constant-current driver is chosen todeliver the desired current, with enough forward voltage output toaccommodate the maximum input voltage of the LED source. LED lightsources are not designed to be driven with a reverse voltage.

By driving LED light sources with a regulated constant-current powersupply the light output variation and lifetime issues resulting fromvoltage variation and voltage changes can be significantly reduced.Therefore, constant current drivers are generally recommended forpowering LED light sources.

Conventional AC-DC power supplies and DC-DC converters provide an outputthat is regulated to provide a “constant-voltage.” However, LEDs workmost efficiently and safest with a “constant-current” drive. LED powersources that provide a “constant-current” output have typically beenreferred to as LED drivers. However, there are heretofore unaddressedneeds with previous solutions for driving multiple LED outputs with asingle inductor in the switching power supply power train, which includecrosstalk and inefficiency.

SUMMARY

Example embodiments of the present disclosure provide systems of drivingmultiple outputs. Briefly described, in architecture, one exampleembodiment of the system, among others, can be implemented as follows: acontrol module configured to supply current from a pulse widthmodulation output configured to drive a plurality of loads to at mostone load of the plurality of loads at one time, the one load selectedbased on its error voltage being the largest of the error voltages ofeach of the plurality of loads.

Embodiments of the present disclosure can also be viewed as providingmethods for driving multiple outputs. In this regard, one embodiment ofsuch a method, among others, can be broadly summarized by the followingsteps: supplying power to a plurality of loads, each load of theplurality of loads producing an error signal; and selecting one load ofthe plurality of loads to supply power to during a single cycle of thepulse width modulation output, the selected load producing a largesterror signal of the plurality of error signals of the plurality ofloads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an example embodiment of a switchingconverter to be used with the disclosed systems and methods of drivingmultiple outputs.

FIG. 2 is a circuit diagram of an example embodiment of a system ofdriving multiple outputs.

FIG. 3 is a circuit diagram of an example embodiment of the switchingFETs of FIG. 2.

FIG. 4 is a flow diagram of an example embodiment of a method of drivingmultiple outputs.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described more fullyhereinafter with reference to the accompanying drawings in which likenumerals represent like elements throughout the several figures, and inwhich example embodiments are shown. Embodiments of the claims may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. The examples set forthherein are non-limiting examples and are merely examples among otherpossible examples.

Most commercially available LED “light modules” are constructed byconnecting a number of LEDs in series or parallel to form cluster orstring configurations. In cases where these light modules include a“constant-current” driver as part of the assembly, an external“constant-voltage” driver or power supply is used. Some LED circuitscontrol the current flowing through the LED with a simple resistor. Thisis another case where a constant-voltage power source is used. Otherexamples where external “constant-voltage” supplies have been employedinclude backlit ad signs, traffic information signs and large screenhigh definition LED displays.

In cases where a manufactured cluster or string of LEDs does not includean internal “constant-current” driver, an external LED driver or powersupply that provides a “constant-current” is used. Constant current LEDdrivers are available in many different package configurations, rangingfrom integrated circuits to enclosed moisture-proof packages, dependingon the application and the required output power.

Depending on the application, LEDs may be connected in series and/orparallel configurations. When LEDs are connected in series, the forwardvoltage drop of each LED in the string are additive. For example, if 15LEDs are configured in series and each one has a voltage drop of 3V (atits nominal current), a voltage source of 45V (15×3V=45V) will drive therequired current (there is also a small additional voltage drop acrossthe sense element). Accordingly, “constant-current” driverspecifications include the output voltage range that it is capable ofproviding to overcome the LED voltage drops. To limit the drive voltageto reasonable levels, multiple strings of series-connected LEDs may beplaced in parallel and driven by multi-output constant-current drivers.

The light output of LEDs may be controlled by varying the amount ofcurrent flowing through the LED (within defined limits) or by turningthe LED on and off via pulse width modulation (PWM). A variable resistormay be used to achieve analog dimming control. In this case, the maximumLED brightness occurs when the resistance is set to its minimum value. Apulse width modulation input may be used to control LED brightness byvarying the duty-cycle of the input signal from 1% to 100%. Typical PWMfrequencies range from 180 to 270 Hz.

LEDs have a relatively quick response time (˜20 nanoseconds), andinstantaneously reach full light output. Therefore, many of theundesirable effects resulting from varying current levels, such aswavelength shift or forward voltage changes, can be minimized by drivingthe light engine at its rated current and rapidly switching that currenton and off. PWM is the best way to achieve stable results forapplications that require dimming to less than 40% of rated current. Bykeeping the current at the rated level and varying the ratio of thepulse “on” time versus the time from pulse to pulse (commonly referredto as the duty cycle), the brightness can be lowered. The human eye cannot detect individual light pulses at a rate greater than 200 cycles persecond and averages the light intensity thereby perceiving a lower levelof light.

In an example implementation of the disclosed systems and methods ofdriving multiple outputs, a single inductor may be used to drivemultiple independent strings of LEDs or white LEDs (WLEDs) such as abacklight driver or a display driver for a mobile device. In an exampleembodiment, a boost DC to DC converter may be used with a singleinductor to drive multiple outputs. The inductor may be larger than thepower control integrated circuit chip, so the fewer the inductors, thebetter. The multiple outputs could be used to drive, for example, akeyboard driver, a camera flash, and a display backlight and driver, andonly one inductor is used. The systems and methods of driving multipleoutputs disclosed herein may be used, as a non-limiting example, todrive a WLED load with advantages of high efficiency and lack ofcross-talk. In an example embodiment, the error voltages of each of themultiple outputs is sampled during each cycle of the DC to DC converterand the largest error voltage is determined for that cycle. Power fromthe DC to DC converter is then supplied to that output during thatcycle.

With cross-talk, noise from one output is reflected on one of the otheroutputs. In multiple output applications, efficiency may be lost becausethe outputs may need different output voltage levels (for exampledriving both a 6 LED string and a 9 LED string). The output voltage ofthe power supply module is set at the highest voltage and the others aredriven through a resistor divider or some other mechanism to lower theoutput voltage, causing a drop in efficiency.

FIG. 1 provides switching converter 100 as an example switchingconverter to be used in the disclosed systems and methods of drivingmultiple outputs. Switching converter 100 is a boost converterconfigured for peak current mode control with synchronous power FETs. Inan example embodiment, low side power FET 150 may be singular, and maybe shared between the multiple output loops. Low side FET sense device155 in combination with an accurate internal sense resistor may be usedto convert the current through inductor 105 during the on-state of lowside FET 150 to a voltage waveform. In an example embodiment, thewaveform is compared to the slop-compensated (in slope compensationamplifier 110) error signal to generate the PWM control signal of thelow side FET. The frequency of operation of the converter may be set viathe PULSEGEN signal into SR latch 130.

The current in inductor 105, sampled through low side FET 155, is usedto charge or discharge into the load by grounding low side FET 150 (lowside FET 150 and sense FET 155 are turned on substantiallysimultaneously). Sense FET 155 feeds current sense amplifier 160,amplifying the voltage across the sense resistor, which providesinformation about the current through the inductor. The amplifiedcurrent sense signal is then fed into PWM comparator 120. Slopecompensation comparator 110 takes the error signal from the output ofthe power module and uses slope compensation to provide stability. Theslope compensated error signal and the current sense signal are used togenerate the PWM signal through SR flip flop 130 and low side driver 140to turn the gate of low side FET 150 on and off.

FIG. 2 provides an example embodiment of the disclosed system andmethods of driving multiple outputs with high side FETs 230, 235, and240. FIG. 2 provides for 3 high side FETS, but any number of high sideFETs could conceivably be used, depending on how many outputs aredesired. LX is the switch node which is connected to the bottom of theinductor at the output of switching converter 100 of FIG. 1. If low sideFET 150 is turned on, one side of inductor 205 is grounded and it ischarged with Vbat in the example circuit of FIG. 2 When low side FET 150is turned off, one of high side FETS 230, 235, and 240 are turned on andthe charge stored in inductor 205 delivers the power to the load. Thefeedback loop works to maintain voltage regulation on the SINKx inputpins at the bottom of LED strings 215, 220, and 225. The SINKx paths canbe independently enabled or disabled, not affecting the operation of anyother SINKx. In an example embodiment, each SINKx input features aregulated current sink and may have independent brightness and dimming(analog and digital) control. These may be accessed via an analog PWMinput, or via a digital register or state machine control. In an exampleemboidiment, the SINKx feedback voltages are fed to summing erroramplifier 265 which generates an error signal proportional to the totalerror of the LED strings.

The back to back nature of the FETS in high side FETs 230, 235, and 240work to prevent leakage from the FET body diodes going back into the LXnode from VOUTx. The charge from inductor 205 is applied to the outputthat has the largest error signal per cycle. In an alternativeembodiment, the charge from inductor 205 is applied to the output thathas the largest percentage error signal per cycle (Verr/VOUTx). In theexample implementation of FIG. 2, there are three D flip flops in flipflop block 250 that are clocked with the PWM signal that is used todrive low side FET 150 of FIG. 1. Although three flip flops are used inflip flop block 250 of FIG. 2, any number of flip flops may be useddepending on the number of outputs that are driven. Data is sent to flipflop block 250 with each clock signal. Arbitration logic block 255decides whether to send a 1 or a 0 to each flip flop of flip flop block250. With each clock cycle, arbitration logic block 255 determines whichof the output voltages sampled at the SINK 1, SINK 2 SINK 3 nodes hasthe most error. Whichever output has the most error in that particularcycle will be the one which will have the power delivered to it. A newdecision is made with each switching cycle. In an example embodiment,only one output path is charged per cycle.

In an example embodiment, arbitration logic 255 is fed by threecomparators (the number of comparators depends on the number of outputsignals to be compared, such that the number of comparators is equal tothe summation of N−x as x goes from 1 to N, where N is the number ofoutput signals). Each comparator of comparator block 260 determines ifone error voltage is higher than each of the other error voltages.Arbitration block 255 determines which output “wins” for that particularcycle and feeds the appropriate digital control to the “winning” flipflop which then gets clocked in and turns on one of high side FET 230,235, and 240 as needed.

In an example embodiment, a user can control the brightness by sinkingmore current. Alternatively, a user can control the brightness with aPWM signal provided by a brightness and dimming control module. To setthe voltages for the outputs of VOUT1, VOUT2, VOUT 3, VREF of 400 mV isused in this example implementation. Then the total error on any numberof outputs is added back in the error signal in error amplifier 265 tocontrol the charging of inductor 205. Since the outputs are unique, thecross-talk between the output loads is decreased. Also, efficiency isoptimized due to using unique output voltages for each string.

FIG. 3 provides circuit 300 of a simplified switch matrix for an exampleembodiment of a system of switching multiple outputs comprising inductor305, low side FET 350 and high side FETs 330, 335, and 340. Inductor 305is charged when low side FET 350 is closed and high side FETS 330, 335,and 340 are open. Power is delivered to a particular output when thatoutput produces the largest error signal. During the power deliverycycle, low side FET 350 is open and one of high side FETs 330, 335, and340 is closed. If VOUT1 produces the largest error signal, high side FET330 is closed and FETs 335, 340, and 350 are open. If VOUT2 produces thelargest error signal, FET 335 is closed and FETs 330, 340, and 350 areopen. If VOUT3 produces the largest error signal, FET 340 is closed andFETs 330, 335, and 350 are open. In an example embodiment, the errorsignals are sampled and compared for each cycle of the PWM signal ofswitching converter 100.

FIG. 4 provides flow diagram 400 of an example embodiment of a method ofdriving multiple outputs. In block 410, power is supplied to a pluralityof loads, each load of the plurality of loads producing an error signal.In block 420, one load of the plurality of loads is selected to supplypower to during a single cycle of the pulse width modulation output, theselected load producing a largest error signal of the plurality of errorsignals of the plurality of loads.

The flow chart of FIG. 4 shows the architecture, functionality, andoperation of a possible implementation of the arbitration logicsoftware. In this regard, each block represents a module, segment, orportion of code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat in some alternative implementations, the functions noted in theblocks may occur out of the order noted in FIG. 4. For example, twoblocks shown in succession in FIG. 4 may in fact be executedsubstantially concurrently or the blocks may sometimes be executed inthe reverse order, depending upon the functionality involved. Anyprocess descriptions or blocks in flow charts should be understood asrepresenting modules, segments, or portions of code which include one ormore executable instructions for implementing specific logical functionsor steps in the process, and alternate implementations are includedwithin the scope of the example embodiments in which functions may beexecuted out of order from that shown or discussed, includingsubstantially concurrently or in reverse order, depending on thefunctionality involved. In addition, the process descriptions or blocksin flow charts should be understood as representing decisions made by ahardware structure such as a state machine.

The logic of the example embodiment(s) can be implemented in hardware,software, firmware, or a combination thereof. In example embodiments,the logic is implemented in software or firmware that is stored in amemory and that is executed by a suitable instruction execution system.If implemented in hardware, as in an alternative embodiment, the logiccan be implemented with any or a combination of the followingtechnologies, which are all well known in the art: a discrete logiccircuit(s) having logic gates for implementing logic functions upon datasignals, an application specific integrated circuit (ASIC) havingappropriate combinational logic gates, a programmable gate array(s)(PGA), a field programmable gate array (FPGA), etc. In addition, thescope of the present disclosure includes embodying the functionality ofthe example embodiments disclosed herein in logic embodied in hardwareor software-configured mediums.

Software embodiments, which comprise an ordered listing of executableinstructions for implementing logical functions, can be embodied in anycomputer-readable medium for use by or in connection with an instructionexecution system, apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions. In the context of this document, a“computer-readable medium” can be any means that can contain, store, orcommunicate the program for use by or in connection with the instructionexecution system, apparatus, or device. The computer readable medium canbe, for example but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, ordevice. More specific examples (a nonexhaustive list) of thecomputer-readable medium would include the following: a portablecomputer diskette (magnetic), a random access memory (RAM) (electronic),a read-only memory (ROM) (electronic), an erasable programmableread-only memory (EPROM or Flash memory) (electronic), and a portablecompact disc read-only memory (CDROM) (optical). In addition, the scopeof the present disclosure includes embodying the functionality of theexample embodiments of the present disclosure in logic embodied inhardware or software-configured mediums.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade thereto without departing from the spirit and scope of theinvention as defined by the appended claims.

Therefore, at least the following is claimed:
 1. A system comprising: aswitching regulator; at most one inductor connected between theswitching regulator and a plurality of loads; and an arbitration logicmodule configured to sample an error voltage of each load of theplurality of loads during each cycle of the switching regulator and tosupply power from the at most one inductor to the load of the pluralityof loads with the largest error voltage of the sampled error voltages.2. The system of claim 1, further comprising a plurality of comparatorsconfigured to compare the feedback of each of the plurality of loadswith each other feedback of the plurality of loads for input into thearbitration logic module.
 3. The system of claim 2, wherein the numberof comparators is at least 2^(n), where n is the number of loads beingdriven.
 4. The system of claim 1, further comprising, for each load: a Dflip flop; a high side driver; and a pair of field effect transistorsconfigured to drive each load.
 5. The system of claim 4, wherein each Dflip flop is clocked by a pulse width modulation signal from theswitching regulator and receives an input from the arbitration logicmodule.
 6. The system of claim 4, wherein the pair of field effecttransistors is configured in a back to back configuration to supplypower from the inductor to one of the plurality of loads.
 7. The systemof claim 1, further comprising a summing error amplifier configured tosum the error voltages of each load together for comparison to areference voltage, the comparison setting the pulse width of the pulsewidth modulation signal of the switching regulator.
 8. The system ofclaim 1, wherein the switching regulator comprises a boost regulator. 9.The system of claim 1, further comprising, for each load: a regulatedcurrent sink configured to sink current from the inductor and acrosseach load, the sampled error voltage sampled between the load and itscurrent sink.
 10. The system of claim 1, wherein at least one loadcomprises at least one white LED.
 11. A switching power supply modulecomprising: a control module configured to supply current from a pulsewidth modulation output configured to drive a plurality of loads to atmost one load of the plurality of loads at one time, the one loadselected based on its error voltage being the largest of the errorvoltages of each of the plurality of loads.
 12. The power supply moduleof claim 11, wherein the error voltages are sampled during each cycle ofthe pulse width modulation output.
 13. The power supply module of claim11, further comprising at most one inductor configured to supply currentto the plurality of loads.
 14. The power supply module of claim 11,further comprising a summing error amplifier configured to sum the errorvoltages of each load together for comparison to a reference voltage,the comparison setting the pulse width of the pulse width modulationoutput.
 15. The power supply module of claim 11, further comprising, foreach load: a regulated current sink configured to sink current from theinductor and across each load, the sampled error voltage sampled betweenthe load and its current sink.
 16. The power supply module of claim 11,further comprising a plurality of comparators configured to compare thefeedback of each of the plurality of loads with each other feedback ofthe plurality of loads for input into the arbitration logic module, thenumber of comparators being at least 2^(n), where n is the number ofloads being driven.
 17. A method of supplying power to a plurality ofloads from a single pulse width modulation output, comprising: supplyingpower to a plurality of loads, each load of the plurality of loadsproducing an error signal; and selecting one load of the plurality ofloads to supply power to during a single cycle of the pulse widthmodulation output, the selected load producing a largest error signal ofthe plurality of error signals of the plurality of loads.
 18. The methodof claim 17, wherein supplying power to the plurality of loads comprisessupplying power to the plurality of loads with at most one inductor. 19.The method of claim 17, further comprising summing the error voltages ofeach load of the plurality of loads together for comparison to areference voltage, the comparison setting the pulse width of the pulsewidth modulation output.
 20. The method of claim 17, wherein at leastone load comprises at least one white LED.